Method for manufacturing tubular films of thermoplastic resins

ABSTRACT

In a method for manufacturing a tubular resinous film wherein a tubular film of a thermoplastic film is extruded from an extrusion nozzle, cooled, heated, expanded and wrapped into a roll, a circumferential twist is intermittently imparted to the tubular resinous film after it is cooled but before it is heated and expanded, or circumferential twists in the opposite directions are imparted alternately, by means of cooperating inner and outer rollers which are revolved in the circumferential direction of the tubular resinous film. Means to cool the extruded tubular resinous film comprises a first stationary cooling cylinder having a diameter slightly smaller than the inner diameter of the tubular resinous film, a second rotary cooling cylinder having a diameter slightly larger than that of the first cooling cylinder and adapted to contact and cool the inner surface of the extruded tubular resinous film, and means to form a layer of cooling gas between the inner surface of the extruded tubular resinous film and the periphery of the first cooling cylinder.

I Umted States Patent [1 1 {111 3,737,495 Nagano et al. [4 1 June 5,1973 54] METHOD FOR MANUFACTURING 3,296,343 1/1967 Buttolph et al.....-..264/95 T B LA FILMS F TgEgMdRPLASTlCoREsINS FOREIGN PATENTS RAPPLICATIONS Inventors: g- Tom-ibfi; 1,444,624 8/1965 France "264/95 iYamataka Primary Examiner-Robert F. White a of Nagahama'sh" shlga'kenAssistant Examiner-Jeffery R. Thurlow Japan Attorney-Wenderoth, Lind &Ponack [73] Assignee: Mitsubishi Jushi Kabushiki Kaisa Chiyoda-Ku,Tokyo-To, Japan ABSTRACT [22] Filed: Sept 29, 1970 In a method formanufacturing a tubular resinous film wherein a tubular film of athermoplastic film is ex- [21] Appl. No.: 76,639 truded from anextrusion nozzle, cooled, heated, ex- 1 panded and wrapped into a roll,a circumferential Related Application twist is intermittently impartedto the tubular resinous [62] Division of 750 036 Aug 5 1968 Pat filmafter it is cooled but before it is heated and ex- 3,635,'634. 1 Ypanded, or circumferential twists in the opposite directions areimparted alternately, by means of [52] Cl "264/95, 264/209 264/210 Rcooperating inner and outer rollers which are revolved 264/237 26 4[3,10 264/348 in the circumferential direction of the tubular resinous[5 Int- Cl. B29d film. Means to Cool extruded tubular resinous [58]Field of Search 264/ 89 237 comprises a first stationary coolingcylinder having a 264/209 103 diameter slightly smaller than the innerdiameter of the tubular resinous film, a second rotary cooling [56]References Cited cylinder having a diameter slightly larger than that ofthe first cooling cylinder and adapted to contact and UNITED STATESPATENTS cool the inner surface of the extruded tubular resinous film,and means to form a layer of cooling gas between 5 822 g 22 3: the innersurface of the extruded tubular resinous film 60 969 251; 21 and theperiphery of the first cooling cylinder. 3,231,651 1/1966 Cheney ..264/6 Chims, 7 Drawing Figures 22 5 MI F 19 r 19 j 1 IV 1V l 11 l l'PATENTEU 51975 SHEET 1 OF 5 FIG.1

FIG.2

HIROSHI ammo, mm Tomm. AKIRA YAMATAKA m HIROHIKO YOSHI'A INVENTORSulldwfiiu vM PATENIELJUH sum 7, 95

sum 2 or 5 FIG? HIROSHI NAGANO. HIDED TOMIOKA.

AKlRA YAMATAM AMI HIROHIKO YOSHIDA INYEN'IDRS BY 01mm PATENTED JUN 51973SHEET u [1F 5 FIG.5

HIROSHI NAGAN HI DEO TDMIOK A. AURA YAMATAM m HIRDHIKO YosmoA.

INVENTORS MflXJMPa-ul.

SHEET 5 BF 5 /A/ I //h PATENTEUJU" 5W HIROsHI M6 N0.

HIDEO TOMIAKA,

AKIIA YAMATAKA mu HI Hmo YosHIM,

INVENTORS BYMJIACMM METHOD FOR MANUFACTURING TUBULAR FILMS OFTHERMOPLASTIC RESINS This application is a division of the applicantscopending patent application Ser. No. 750,036, filed Aug. 5, 1968, U.S.Pat. No. 3,635,634.

BACKGROUND OF THE INVENTION This invention relates to a method formanufacturing tubular films of thermoplastic resins. More particularly,this invention relates to an improved method whereby it is possible touniformly take up a tubular resinous film extruded from an extrusionnozzle and fold it into a flat sheet without forming an annular bulge orraised portion in the wrapped coil.

With recently increased demand for films of synthetic resins(hereinafter sometimes called plastic film), these films are oftenprinted with letters or patterns or are used for automatic packaging.For these reasons, the conditions for the quality of plastic films arebecoming more strict, and a pressing requirement of such films is thatthey be free of any local slack or wrinkle.

The manufacturing technique for plastic films has also made asignificant advance. More particularly, an economical method ofmanufacturing plastic films has been proposed wherein a thermoplasticresin is extruded into a tubular form, the tube is then expanded andelongated by blowing air therein, folded into a flat sheet, and thenwrapped into a roll. However, this method inevitably results in uneventhickness of the film due to non-uniformity of the slit width of theextrusion nozzle. As such uneven thickness is formed at a definiteportion of the film depending upon the extrusion nozzle employed, whensuch a film with uneven thickness is wrapped into a roll, a bulge orraised portion is formed at definite portion along the periphery of theroll. The material of the resin at the raised portion is subjected to atensile stress higher than those at other portions and is thereforeelongated. Consequently, when the film is payed out from the roll, theportion of the film corresponding to the raised portion slackens, thuscausing various troubles during use of the film. This is particularlydetrimental when the film is printed or supplied to an automatic packingmachine and the like. The effect of the bulge is more pronounced in thecase of pliable films. Furthermore, in the case of heat shrinkablefilms, since there is a tendency to create a slight shrinkage, slack inthe material may become so large that it precludes practical use of thematerial.

Thus, uneven thickness of the film causes much trouble, but control ofthe film thickness requires highly skilled technique and time. Even withminute care, it is difficult to completely eliminate uneven thickness,thus decreasing the yield of acceptable products.

However, if some means could be provided by which slack can be preventedat the time of application of the film, uneven thickness would not causeany difficulty.

When wrapping a plastic film into a roll, if portions of uneventhickness could be distributed along the length of the roll, the bulgewould not be formed, thus obviating creation of local slacks in thefilm. According to one proposed method of distributing portions ofuneven thickness, an extruding machine or an extrusion nozzle is rotatedwhereby the tubular film is taken up while it is twisted in thecircumferential direction. However, the mechanism of such a rotaryextruder is extremely complicated so that such a method cannot beapplied to large size machines.

Further, with such a rotary extrusion nozzle the life of the machine isshortened due to the friction between the rotary extrusion nozzle andthe machine support therefor. Heat of friction also decomposes theresin. Thus, the machine of this type is not suitable for the continuousoperation over a long time.

In the manufacture of tubular films of thermoplastic resins, it is usualto extrude a resinous tubular member in the molten state, cool thetubular member and then elongate it from 1.5 to 10 times in both thelongitudinal and transverse directions by a suitable operation. Thecooling process for the molten tubular member has a great effect uponthe subsequent elongation process and the characteristics of the tubularfilm after elongation. For this reason, in highly crystallinethermoplastic resins such as crystalline polypropylene, it is highlydesirable to cool the molten tubular body in an extremely short periodin order to prevent crystallization. Cooling of the molten tubular bodyhas been performed by either directly cooling the outer surface of thetubular body by means of a pressurized gas or cooling liquid or byproviding a water cooled jacket in the tubular body. Of these methods,the method wherein the outer surface alone is cooled cannot rapidly coola tubular body having a relatively larger wall thickness prior toelongation. Moreover cooling is'not uniform, thus causing rapidcrystallization of the resin. This not only causes trouble in subsequentelongation process but also impairs the optical characteristics of thetubular film after elongation. To cool both the inner and outer surfacesof the tubular body, these two surfaces are simultaneously cooleddirectly by a cooling liquid or the inner surface is cooled by means ofa water cooled jacket while the outer surface is cooled by fallingwater. However, each of these prior methods requires complicatedapparatus. In addition, control of the cooling liquid is not easy.Especially, with the first method or the method wherein both the innerand outer surfaces are cooled by a cooling liquid, there are suchdefects that complete removal of the cooling liquid from the interior ofthe tubular body is difficult and that fluctuation of the liquid levelresults in non-uniform cooling. Accordingly, effective means for coolingextruded plastic film has been long desired.

SUMMARY OF THE INVENTION We have discovered a novel method of preventingraised portions from being formed in a roll of an extruded tubularresinous film by twisting the film prior to the wrapping thereof into aroll, thus distributing raised portions without the necessity ofrotating an extruder or an extrusion nozzle. The method of imparting atwist to the tubular resinous film is not novel. For example it has beenproposed to provide a plurality of rollers which have axes parallel tothe direction of extrusion of the tubular resinous film and disposed toengage the outer surface of the bulge, the inner surface thereof beingunder pressurized air. By rotating these rollers the friction createdbetween rollers and the tubular resinous film functions to twist thefilm. With this method, however, unless all rollers for moving theresinous film have the same speed, wrinkles are formed in the resinousfilm before it is wrapped into a roll, thus resulting in non-uniformwrapping. Theoretically, it would be possible to impart the desiredtwist to the tubular resinous film without forming any wrinkle if allrollers were designed to have equal dimensions and rotational speed.However, in actual practice, the speeds of respective rollers effectiveto move the resinous film become slightly different due to such causesas slight variation in the air pressure acting upon the inner surface ofthe tubular resinous film, the surface condition of rollers, vibrationof rollers, slip, and so forth.

- It is therefore an object of this invention to provide a novel methodfor manufacturing tubular resinous films which can circumferentiallytwist the films and wrap them into rolls without forming wrinkles.

Another object of this invention is to provide a novel method formanufacturing tubular resinous films which can impart twist to tubularresinous films extruded from extrusion nozzles without forming anywrinkle and can wrap them into rolls uniformly at high speeds.

Still another object of this invention is to provide a novel coolingmethod for cooling tubular resinous films as they are extruded from theextrusion nozzle.

According to one aspect of the invention, these objects can be attainedby providing a method whereby it is possible to impart circumferentialtwist to a tubular resinous film extruded from an extrusion nozzle, saidmethod including the operation of a plurality of inner rollers havingaxes of rotation substantially perpendicular to the direction ofextrusion of said tubular resinous film from said extrusion nozzle andmaintained in contact with the inner surface of said tubular resinousfilm, and a plurality of outer rollers cooperating with respective innerrollers and having axes of rotation substantially parallel to those ofcorresponding inner rollers, both of said inner and outer rollers beingdisposed adjacent said extrusion nozzle and being revolved at the sameangular speed in the circumferential direction of the tubular resinousfilm, thus twisting the same in the circumferential direction.

We have found that among various prior methods of wrapping a tubularresinous film into a roll, the method wherein the tubular film is foldedinto a flat sheet and then the sheet wrapped into a coil while thetubular resinous film is twisted in the circumferential direction ismost economical.

Accordingly, a still further object of this invention is to provide animproved method of manufacturing a tubular resinous film which candistribute the portion of the film of uneven thickness along the lengthof a coil without forming any wrinkle in the film.

Thus, another aspect of this invention relates to an improvement of amethod of continuously manufacturing a tubular resinous film whichcomprises the steps of cooling a tubular resinous film extruded from anextrusion nozzle, heating the tubular resinous film, expanding thetubular resinous film by the air pressure admitted therein, and foldingsaid expanded tubular resin film into a flat sheet, said improvementbeing characterized by alternately repeating a step of twisting thetubular resinous film in the circumferential direction for a firstinterval and a step of stopping the twisting process for a secondinterval. Advantageously, these first and second intervals are madesubstantially equal to the interval during which the extruded tubularresinous film travels from the extrusion nozzle to a point at which thefilm begins to expand.

Alternatively, circumferential twists are imparted alternately in theopposite directions with a third interval interposed therebetween,during which the twisting operation is interrupted.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. ll shows a diagrammatic side elevation to explain the principle ofthe present invention;

FIG. 2 is a plan view of the system shown in FIG. 1;

FIG. 3 is a diagrammatic side elevation to explain the operation of thepresent invention;

FIG. 4 is a plan view of one embodiment of this invention with a part insection generally taken along the line IVIV in FIG. 3;

FIG. 5 is an enlarged view of a portion of the apparatus shown in FIG.4;

FIG. 6 is a graph to explain the operation of the present invention; and

FIG. 7 is a sectional view of cooling means according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2 ofthe accompanying drawing, a cylindrical resinous film 10 is extrudedfrom an annular extrusion nozzle 11 of an extruding machine. Inside thefilm are provided a plurality of inner rollers 12 having axesperpendicular to the direction of movement of the film (indicated byanarrow A) and mounted on an inner support 13 to be urged against theinner surface of the film.

. Inner support 13 is mounted on a rotary shaft 14 which also carries arotary cooling former 15, and which is journalled by a stationarycooling former 16 having a smaller diameter than the rotary former sothat inner rollers 12 can revolve in the circumferential direction alongthe inner surface of tubular resinous film 10.

These rotary and stationary formers l5 and 16 are in the form of hollowcylinders and are cooled by water, the rotary former being disposed tocontact the inner surface of the resinous film. Whereas stationaryformer 16 is secured to extrusion nozzle 11, and since its diameter isslightly smaller than that of the rotary former, there is a small gapbetween the periphery of the stationary former and the tubular resinousfilm. Cooling gas such as air is admitted through an inlet pipe 17 anddischarged through an outlet pipe 18 to form a thin film of cooling gasin the gap. A plurality of outer rollers supported by an outer support20 are provided to contact against the outer surface of tubular resinousfilm 10, each of these outer rollers 19 being opposed to a correspondingone of inner rollers 12 and has an axis substantially parallel to theaxis of the corresponding inner roller. As shown, four pairs ofcooperating inner and outer rollers are provided, each located on oneside of a square to clamp the resinous film therebetween. Accordingly,when rotary shaft 14 and outer support 20 are revolved in thecircumferential direction of the resinous film at the same angularspeed, the film is twisted circumferentially. As rollers 12 and 19 arerotated in the direction of extrusion A of the film. the revolution doesnot interfere with the movement of the film. Further, since the film isclamped between inner and outer rollers and since all rollers arerevolved simultaneously, there is no slip between rollers and theresinous film, thus preventing any wrinkle from being formed in theresinous film. Stationary cooling former l6 cools the tubular resinousfilm immediately after extrusion thereof from the extrusion nozzle.Rotary cooling former 15 which is rotated with inner rollers 12effectively cools the tubular resinous film while at the same timeassisting twisting thereof. This cooling effected in two stagesincreases the extrusion speed of the resinous film, thus enabling highspeed manufacturing of tubular resinous films.

FIG. 3 is a diagrammatic side view, partly in section, showing the stateof tubular resinous film l extruded from extrusion nozzle 11 of anextruder and wrapped around a take-up roll. Tubular resinous film 10 iscooled by stationary cooling former 16 and rotary cooling former 15immediately after it has been extruded from the extrusion nozzle, thentwisted in the circumferential direction by inner and outer rollers 12and 19, heated and softened by the heat applied by a heater 22 andexpanded and elongated by air supplied from inlet pipe 17. Finally theexpanded tubular resinous film 10 is folded into a flat sheet by meansof pinch rollers 21a and is then wrapped around a take-up roll 21.

Even when portions of uneven thickness are formed in the extruded filmdue to the characteristic of the extrusion nozzle 11, their positionsare definite, and by the action of twist imparted to the film in themanner as described above, such thick portions are uniformly distributedalong the length of the take-up roll, thus eliminating the bulge in theroll of the film.

FIGS. 4 and 5 show a preferred embodiment of the apparatus according tothis invention.

In FIGS. 4 and 5, reference numerals 12, 19, 20 10, and 14 designate thesame parts as above. As shown, inner rollers 12 are mounted on innersupport 13 with their axes substantially perpendicular to the directionof movement of tubular resinous film to engage the inner surfacethereof. At its center, inner support 13 is secured to the upper end ofrotary shaft 14 which is journalled by stationary cooling former 16secured to the extrusion nozzle. As shown in FIG. 4, an electric motor23 is provided to drive four outer rollers 19 at the same speed to drivethe resinous film in the direction perpendicular to the sheet of thedrawing through a speed reduction mechanism 24, a belt 25, a shaft 26,and bevel gears 27. All of these components are mounted on outer support20. Outer rollers 19 are supported by bearing pedestals 28 secured tosupport 20.

Support 20 is in the form an annulus surrounding the tubular resinousfilm and is supported by a stationary frame 29 through ball bearings 30.A gear 31 is formed on the periphery of the annular support 20 to meshwith a pinion 32 mounted on a shaft 33 which is driven by an independentdriving device (not shown) whereby the support 20 is rotated in thedirection of arrows B or in the circumferential direction of the tubularresinous film which is clamped between pairs of outer and inner rollers19 and 12 and driven by the former.

Rotation of shaft 33 causes outer support 20 to rotate about the tubularresinous film, thus revolving outer rollers 19 supported thereby in thecircumferential direction of the tubular resinous film. Consequently,inner rollers 12 as well as inner support 13 are revolved about the axisof shaft 14 at the same angular speed as the outer rollers, thustwisting the resinous film in the circumferential direction while it ismoving in the direction of arrow A. To adjust the gap between inner andouter rollers, hand wheels 34 are provided.

In this manner, tubular resinous film 10 extruded from the extrusionnozzle is twisted in the circumferential direction before it is heatedand expanded. After expansion the film is folded into a flat sheet bypinch rollers 21a and finally wrapped into a roll about takeup roll 21.As the circumferential twist is imparted to the tubular resinous film byrevolving inner and outer rollers which clamp the film therebetween andhaving axes substantially perpendicular to the direction of extrusion ofthe film, such revolution does not interfere with the extrudingoperation of the resinous film. Further, incorporation of the rotary andstationary formers into the twisting mechanism enables high speedproduction of tubular resinous films. It should be understood that thenumber of pairs of inner and outer rollers is not limited to four, butany suitable number may be selected depending upon the diameter of theextrusion nozzle.

According to modified methods, twist is imparted intermittently ortwists in opposite directions are imparted alternately to the tubularresinous film. According to the latter method, the tubular resinous film10 is twisted in one direction by revolving inner and outer rollers 12and 19 for an interval t which is equal to the time interval requiredfor the film to travel from extrusion nozzle 11 to a point M which issurrounded by heater 22 and at which the film begins to expand.Thereafter, twisting is interrupted for the same interval t, that is thefilm is advanced without revolving inner and outer rollers. Then twistis imparted in the opposite direction for the same period by revolvingthe inner and outer rollers in the opposite direction. Thereaftertwisting is interrupted for the same period. This cycle of operation isrepeated.

As the film is soft immediately after it is extruded from the extrusionnozzle and also at point M the film is twisted very readily withoutforming wrinkles, thus distributing portions of the film of uneventhickness along the axial length of the take-up roll.

FIG. 6 illustrates one example of the twisting process. In this figure,the abscissa represents time T, the ordinate represents twist angle 0(it is assumed that the cross-sectional configuration of tubularresinous film 10 is circular), solid lines indicate the shift angle ofthe portion of uneven thickness of the film when it passes throughexpansion initiation point M, and dotted lines indicate the shift angleof support for outer rollers. Twist is continuously imparted to the filmfor a first interval t interrupted for a second interval t and impartedin the opposite direction for a third interval t these intervals beingsubstantially equal to the time in-' terval t during which the filmtravels from the extrusion nozzle to the expansion initiating point M.Thus, 1,, is L Angle 0,, is determined in accordance with therelationship between the extrusion speed (usually constant) of thetubular resinous film and angular speed to of twist and can be expressedby 0,, =0: X t,,.

As the film extruded from the extrusion nozzle is continuously twistedin one direction for a definite period, the portion thereof havinguneven thickness is gradually shifted in the circumferential directionso that said portion passes point M at successively different angularpositions. After passing through expansion initiating point Msubstantially no twist is applied to the film so that portions of uneventhickness are wrapped at the same angular positions as at point M.

After the elapse-of interval t twist is interrupted, whereby aparticular portion of uneven thickness formed by the extrusion nozzlewill not be shifted circumferentially, and the length of the tubularresinous film between the extrusion nozzle and the expansion initiatingpoint has already been twisted. Then, at point M, this particularportion will be at the same angular position as it is formed by theextrusion nozzle.

After the elapse of interval t twist is imparted in the oppositedirection for interval t thus gradually shifting portions of uneventhickness in the opposite direction. After the elapse of interval t,,,the twisting operation is stopped. Then, when passing through theexpansion initiating point, portions of uneven thickness will begradually restored to their original positions, that is, positions atwhich they are initially formed.

In this manner, portions of uneven thickness are shifted in onedirection and then in the other direction by the above described cycleof operation including the steps of twisting in one direction,interrupting twisting, twisting in the opposite direction, andinterrupting twisting.

The circumferential length of the tubular resinous film along whichportions of uneven thickness are distributed can be expressed by 2 0,,(radians). Thus, for example where 6, IT/2 radians, 11/2 said length isequal to 1r radians. In other words, portions of uneven thickness aredistributed over the entire width of the folded sheet wrapped aroundtake-up roll 21. In this way, uniformly wrapped rolls can be formedwithout creating any wrinkle.

Although angular speed In at which the twist is imparted is determinedessentially in accordance with the take-up speed V of the resinous film,it also depends upon the type of the resin. For example tubular films ofpolyethylene are ordinarily taken-up at a speed of from about 5 to 9.5m/sec. If in this case twist is imparted at a speed exceeding rpm, careshould be taken not to form wrinkles at softened portions of the tubularresinous film, for example, at expansion'initiating point M.

Alternatively, twist can be imparted by a cycle of operation includingthe steps of twisting and interrupting twisting. In this modifiedmethod, the twisting process in the opposite direction is eliminated. Inthis case, also, it is possible to distribute portions of uneventhickness along the entire width of the folded film by so adjustingangular speed 0 of twist that 0,, becomes equal to 11 radians. Althoughit has heretofore been difficult to twist tubular resinous films, thisinvention has succeeded in solving this problem.

The following are examples of the novel method.

Example 1 When extruding a tubular film of heat shrinkable vinylchloride resin, the portion of uneven thickness was marked with an inkimmediately after it was extruded from the extrusion nozzle.Circumferential shift was observed by the shift of the ink mark, theamount of shift being expressed by an angle subtended by the shiftedlength.

Parameters utilized were as follows. Feed speed of rollers 19 was8m/min. Take-up speed of the film after expansion was l8m/min.- Theratio of diameter of the extrusion nozzle to the diameter of the tubularresinous film after expansion was 122.5. Under these conditions thedirection of twist was reversed at a definite period. Twisting speed was1.67 rpm, twist was imparted by a cycle including twisting in onedirection for 18 seconds,

interrupting twist for 18 seconds, twisting in the opposite directionfor 18 seconds at the same twisting speed, and interrupting twisting for18 seconds.

The ink mark applied to the portion of uneven thickness at the exit endof the extrusion nozzle shifted gradually from the center of the take-uproll toward its one end, then shifted to the opposite end and finallyshifted back to the center. Thus portions of uneven thickness weredistributed over a width of 360. By repeating the above cycle ofoperation, a roll of film was formed having no bulge and having uniformdiameter throughout the entire length.

Example 2 As in example 1, an ink mark was applied to a portion ofuneven thickness of a film. In this case, the feed speed of rollers 19was Sm/min, the take-up speed of the film after expansion was l2m/min,the ratio of extrusion nozzle diameter to the diameter of the tubularresinous film after expansion was 1:24, and twist in one direction wasimparted at'a definite period. Twisting speed was 2 rpm. Twist wasimparted for 15 seconds and then stopped for 15 seconds. The ink markshifted gradually from one end to the other of the take-up roll and thenreturned to the original position. Thus the ink mark was distributedover a width of 180. By repeating the above described cycle, a roll ofresinous film having uniform diameter and free from any bulge wasobtained.

Example 3 The same process as in example 2 was followed except that thetwisting speed was increased to 4 rpm. As a consequence portions ofuneven thickness were distributed over a width of 360.

FIG. 7 shows a detailed sectional view of an extrusion nozzle and watercooled formers.

An annular extrusion nozzle 51 is provided through an extrusion head 50,and a resin inlet port 52 leading to extrusion nozzle 51 is connected toan extruder, not shown. A first cooling cylinder or former 53 is securedto extrusion head 50 on the exit end of the nozzle and a second coolingcylinder or former 54 having a diameter slightly larger than the firstcylinder is provided above it. The second cooling cylinder is rotatablysupported by the first cooling cylinder. These cylinders are providedwith water jackets 53a and 54a, respectively, through which coolingwater is circulated through inlet pipes 55 and outlet pipes 56. Airunder pressure is introduced to flow around the periphery of the firstcylinder through an air inlet pipe 58 and openings 57 through the-bottomwall of the second cylinder and is discharged through outlet pipes 59. Atubular resinous film 10 extruded by extrusion nozzle 51 is cooled bythe first and second cooling cylinders and is then twisted and expandedin the manner as above described. Expansion is effected by the airsupplied through a pipe 60. As described before, the first coolingcylinder 53 has a diameter slightly smaller than that of the secondcooling cylinder 54 and an air film is formed around the first coolingcylinder. The ratio of outer diameters of the first and second coolingcylinders and the pressure of air supplied through inlet pipe 58 are soselected that the tubular resinous film will not contact withtheperiphery of the first cylinder when the film contracts by beingcooled by the pressurized air, but will make light contact with theperiphery of the second cooling cylinthat the resinous film is rapidlycooled while it is still soft. This results in an increase in thetake-up speed of the resinous film. It was found that the take-up speedwas increased 1.5 to 3 times when compared with that of the conventionalcooling device.

As mentioned above, when manufacturing a tubular film of a thermoplasticresin by the method described above, that is, by a method comprising thesteps of cooling the extruded tubular resinous film, heating the cooledfilm, and expandingand elongating the film, cooling before commencingexpansion and elongation is an important factor that determines thephysical characteristics, especially the optical characteristic such aslight transmissibility, of the product. The above described coolingdevice was found to improve the physical characteristics of the finishedresinous film.

While the invention has been described in terms of preferred embodimentsthereof, it will be obvious to one skilled in the art that many changesand modifications may be made therein without departing from the scopeof the invention as defined in the appended claims.

We claim:

1. A method of manufacturing a tubular resinous film comprising thesteps of extrudinga tubular resinous film from a stationary extrusionnozzle; cooling said resinous film to a solid state immediately after ithas been extruded; imparting a circumferential twist to said tubularresinous film for a first predetermined period after said cooling bypassing said resinous film between a plurality of cooperating pairs ofinner and outer rollers, the axes of rotation of which are substantiallyperpendicular to the direction of extrusion of said resinous film,rotating said rollers in contact with said film, and simultaneouslyrevolving said inner and outer rollers circumferentially of saidresinous film; interrupting said twist for a second predetermined periodby discontinuing said revolving; said steps of imparting the twist andinterrupting the twist being repeated alternately; heating said twistedresinous film to a stretchable state; expanding said heated tubularresinous film to have a diameter 1.5 to 10 times as great as that of thetubular resinous film immediately after said extrusion by introducingpressurized gas into said tubular resinous film; folding said expandedresinous film into a flat sheet; and wrapping said sheet into a roll;said first and second predetermined periods being substantially equal tothe interval during which the extruded tubular resinous film travelsfrom said extrusion nozzle to a point at which said heated tubular filmbegins to expand.

2. The method according to claim 1, wherein said circumferential twistis made at an angle 6m which is expressed by Om m X t where (nrepresents the angular speed of twist and t said first predeterminedperiod.

3. The method according to claim 1 whereinsaid cooling is effected intwo stages, in the first stage the tubular resinous film is cooled by astream of gas flowing through an annular space between the inner wall ofsaid tubular resinous film and a stationary cooling former and in thesecond stage the tubular resinous film is cooled by direct contact witha rotary cooling former. 4

4. A method of manufacturing a tubular resinous film comprising thesteps of extruding a tubular resinous film from a stationary extrusionnozzle; cooling said resinous film to a solid state immediately after ithas been extruded; imparting a circumferential twist in a firstdirection to said tubular resinous film for a first predetermined periodafter said cooling by passing said resinous film between a plurality ofcooperating pairs of inner and outer rollers, the axes of rotation ofwhich are substantially perpendicular to the direction of extrusion ofsaid resinous film, rotating said rollers in contact with said film, andsimultaneously revolving said inner and outer rollers circumferentiallyof said resinous film; interrupting said-twist for a secondpredetermined period by discontinuing said revolving; imparting acircumferential twist in the opposite direction to said tubular resinousfilm for a third predetermined period by reinitiating said revolving inthe opposite circumferential direction; said steps of imparting thetwist in said one direction, interrupting the twist and imparting thetwist in said opposite direction being repeated cyclically; heating saidtwisted resinous film to a stretchable state; expanding said heatedtubular resinous film to have a diameter 1.5 to 10 times as great asthat of the tubular resinous film immediately after said extrusion byintroducing pressurized gas into said tubular resinous film; foldingsaid expanded resinous film into a flat sheet, and wrapping said sheetinto a roll; said first, second and third predetermined periods beingsubstantially equal to the interval during which the extruded tubularresinous film travels from said extrusion nozzle to a point at whichsaid heated tubular resinous film beings to expand.

5. The method according to claim 4, wherein said circumferential twistis made at an angle 0m which is expressed by 0m m X 2,, where 0:represents the angular speed of twist and t R said first predeterminedperiod.

6. The method according to claim 4 wherein said cooling is effected intwo stages, in the first stage the tubular resinous film is cooled by astream of gas flowing through an annular space between the inner wall ofsaid tubular resinous film and a stationary cooling former and in thesecond stage the tubular resinous film is cooled by direct contact witha rotary cooling former.

i l I l

2. The method according to claim 1, wherein said circumferential twistis made at an angle theta m which is expressed by theta m omega X tRwhere omega represents the angular speed of twist and tR said firstpredetermined period.
 3. The method according to claim 1 wherein saidcooling is effected in two stages, in the first stage the tubularresinous film is cooled by a stream of gas flowing through an annularspace between the inner wall of said tubular resinous film and astationary cooling former and in the second stage the tubular resinousfilm is cooled by direct contact with a rotary cooling former.
 4. Amethod of manufacturing a tubular resinous film comprising the steps ofextruding a tubular resinous film from a stationary extrusion nozzle;cooling said resinous film to a solid state immediately after it hasbeen extruded; imparting a circumferential twist in a first direction tosaid tubular resinous film for a first predetermined period after saidcooling by passing said resinous film between a plurality of cooperatingpairs of inner and outer rollers, the axes of rotation of which aresubstantially perpendicular to the direction of extrusion of saidresinous film, rotating said rollers in contact with said film, andsimultaneously revolving said inner and outer rollers circumferentiallyof said resinous film; interrupting said twist for a secondpredetermined period by discontinuing said revolving; imparting acircumferential twist in the opposite direction to said tubular resinousfilm for a third predetermined period by reinitiating said revolving inthe opposite circumferential direction; said steps of imparting thetwist in said one direction, interrupting the twist and imparting thetwist in said opposite direction being repeated cyclically; heating saidtwisted resinous film to a stretchable state; expanding said heatedtubular resinous film to have a diameter 1.5 to 10 times as great asthat of the tubular resinous film immediately after said extrusion byintroducing pressurized gas into said tubular resinous film; foldingsaid expanded resinous film into a flat sheet, and wrapping said sheetinto a roll; said first, second and third predetermined periods beingsubstantially equal to the interval during which the extruded tubularresinous film travels from said extrusion nozzle to a point at whichsaid heated tubular resinous film beings to expand.
 5. The methodaccording to claim 4, wherein said circumferential twist is made at anangle theta m which is expressed by theta m omega X tR where omegarepresents the angular speed of twist and t R said first predeterminedperiod.
 6. The method according to claim 4 wHerein said cooling iseffected in two stages, in the first stage the tubular resinous film iscooled by a stream of gas flowing through an annular space between theinner wall of said tubular resinous film and a stationary cooling formerand in the second stage the tubular resinous film is cooled by directcontact with a rotary cooling former.